In the prior art, ceramic extrusion molding compositions use binders such as starch, polyacrylic acid, polyethylene oxide, alkyl celluloses, hydroxyalkyl celluloses, and hydroxyalkyl alkyl celluloses in aqueous solution form. Of these, alkyl celluloses and hydroxyalkyl alkyl celluloses are in stark contrast to other water-soluble binders having the drawback that aqueous solutions thereof lose an apparent viscosity upon heating so that the bound shape may fail or alter during drying, in that the alkyl celluloses and hydroxyalkyl alkyl celluloses are able to retain the bound shape because their aqueous solutions become gel upon heating. Thus drying proceeds in the state bound by the gel. This binding enables to maintain the molded shape. For this reason, the alkyl celluloses and hydroxyalkyl alkyl celluloses are used in ceramic molding, for example, as the binder having a shape retaining ability during drying.
If the binder forms a gel upon heating, shape retention is satisfactory, but water is trapped within the gel. This interferes with evaporation of water and retards drying. It would thus be desirable to have a water-soluble binder having a shape retaining ability during drying and offering a high drying rate. Furthermore, when the binder is added to a ceramic material in such an amount as to develop a thermal gel strength necessary in the drying step, there arises a problem that the binder provides a high viscosity and causes sticking to the screw and die during extrusion molding, increasing the extrusion pressure. This prevents the ceramic material from being quickly extruded through the die.
Attempts were made to overcome these drawbacks by changing the degree of polymerization or substitution of cellulose ether to tailor its excess viscosity, or by using various organic additives. None of these attempts are fully satisfactory.